1. Field of the Invention
The present invention relates to an antenna device that operates in at least two frequency bands on a low frequency side and a high frequency band higher than the frequency bands on the low frequency side.
2. Description of the Related Art
International Publication No. 2010/113353 discloses a multiband antenna device. The antenna device includes a reactance changing section and a matching section. The reactance changing section is connected in series to a radiating element, and the matching section is shunt-connected between a feeding circuit and the reactance changing section. The reactance changing section and the matching section are each defined by a parallel resonant circuit including an inductor and a capacitor.
The antenna device can support multiple bands by varying the resonance frequency of the reactance changing section. The matching section performs impedance matching.
By varying the resonance frequency of the reactance changing section, the radiating element can be operated in a band selected from multiple operating bands. In this case, in operating bands other than the selected one, a high return loss of the radiating element makes it difficult to perform efficient operation. For example, when it is desired to allow the radiating element to operate in both a band selected from multiple low frequency bands and a high frequency band, varying the resonance frequency of the reactance changing section to switch the low frequency band affects the high frequency band. This may lead to a loss of impedance matching in the high frequency band.